1. Field of the Invention
The present invention relates to a copolymer useful for positive photoresist and a chemical amplification type positive photoresist composition comprising the same. More particularly, the present invention relates to an amide-grafted polyhydroxyphenol resin and a photoresist composition which allows for a good pattern even though a post-baking process is carried out in a delayed time.
2. Description of the Prior Art
The high integration of semiconductor devices has been partly but critically based on a significant advance in lithography. For example, ultra-fine patterns as small as sub-microns or quarter-microns in size must be required for the fabrication of ultra-LSI. Accordingly, the light sources used to form the fine patterns become shorter and shorter in wavelength, for example, from g-line to i-line and deep uv light, further to KrF excimer laser, and finally to electron beams.
Since the photoresists utilizing g-line or i-line, which are largely of novolak-quinones, show large absorption peaks at the wavelength range of deep uv light and excimer laser, a fine pattern is not obtainable therefrom. Thus, there was a strong demand for a material which absorbs little of the light belonging to such wavelength ranges. In response to the demand, active research has been directed to the development of the chemical amplification photoresists based on polyhydroxystyrene derivatives, which are smaller in absorbance at such ranges than are novolak-quinonediazides.
A chemical amplification photoresist consists mainly of a base resin, a compound which generates acid upon radiation(hereinafter referred to as "photoacid generator") and a solvent dissolving them and, optionally, additives. For example, U.S. Pat. No. 4,491,628 discloses a chemical amplification photoresist employing t-butoxycarboxy-grafted polyhydroxystyrene as a base resin.
Also, there are disclosed many preparing methods for chemical amplification photoresist, including, for example, prevention of resist from being in contact with the base in the air (J. Photopolym. Sci. Technol., 8(1995) 519), post- and pre-baking at high temperatures lest the base in the air should be absorbed in a resist film (J. Photopolym. Sci, Technol., 9(1996) 557), introduction of acetal group to allow the physical and chemical properties of photoresist to change with weak acids (J. Photopolym. Sci, Technol., 9(1996) 611) and the addition of additives (J. Photopolym. Sci, Technol., 9(1996) 677).
In practice, however, satisfactory results could not obtained from the conventional resins and by the above techniques. The conventional chemical amplification photoresists are very poor in resolution and thermal resistance and, in particular, they cannot be developed into desirable patterns if a post-baking process is delayed.